Mechanism-based inhibition (MBI) of CYP enzymes, as seen with erythromycin and CYP3A4, differs from competitive inhibition in its clinical consequences. What is the defining characteristic of MBI?

• A MBI is concentration-dependent and reversible upon drug washout
• B MBI involves oxidation of the inhibitor by the CYP enzyme to a reactive intermediate that irreversibly inactivates the enzyme active site, requiring new enzyme synthesis for recovery — a time-dependent inhibition ✓
• C MBI inhibits CYP enzyme synthesis at the transcriptional level rather than at the protein level
• D MBI competitively inhibits multiple CYP isoforms simultaneously without reactive intermediate formation

Explanation

Mechanism-based inhibition (MBI), also called time-dependent inhibition (TDI) or suicide inhibition, occurs when the CYP enzyme first oxidizes the inhibitor drug to a reactive metabolite intermediate (typically a nitroso or nitrosoalkane intermediate for erythromycin/macrolides, a reactive carbene for clopidogrel, or other reactive species). This reactive intermediate forms a stable, irreversible (or quasi-irreversible) complex with the heme iron or the apoprotein of the CYP enzyme, permanently inactivating it. Recovery requires synthesis of new CYP enzyme protein — typically 1–3 days. This explains why macrolide CYP3A4 drug interactions can persist for days after stopping the macrolide, and why the inhibition worsens with continued dosing (progressive enzyme inactivation). This differs fundamentally from reversible competitive inhibitors whose effects resolve with drug elimination.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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